The field of the invention relates to fuel control systems for fuel injected engines which include fuel vapor recovery systems. In particular, the invention is applicable to fuel injected engines wherein the intake air/fuel ratio is regulated via a feedback loop from an exhaust gas oxygen sensor (EGO).
Feedback control of fuel injected engines is known. Typically, mass airflow inducted through the engine is measured and a corresponding desired fuel charge calculated which corresponds to a desired air/fuel ratio. In response, the pulse width of an electronic signal applied to the fuel injectors is varied in an effort to achieve the desired fuel charge. A feedback loop responsive to an exhaust gas oxygen sensor (EGO) further trims the pulse width such that the actual air/fuel ratio approaches the desired air/fuel ratio. The injectors are manufactured to close tolerances such that the relationship of fuel delivered to pulse width is reasonably linear over the operating range of the engine (idle to full load), otherwise, accurate air/fuel ratio control is not achievable.
Fuel vapor recovery systems are also known wherein a portion of evaporative fuel vapors from the fuel system are absorbed in a vapor recovery canister, typically containing activated charcoal, to prevent discharge of fuel vapors into the atmosphere. Under certain engine operating conditions, usually when inducted mass airflow is above a threshold value, ambient air is inducted through the canister into the engine intake, a condition referred to as purging. During a purge cycle, evaporative fuel vapors may also be inducted directly into the engine from the fuel system.
It is also known to combine feedback control systems with fuel vapor recovery systems. For example, U.S. Pat. No. 4,013,054 issued to Balsley et al and U.S. Pat. No. 3,963,009 issued to Mennesson disclose a fuel vapor recovery system coupled to the engine intake via an electronically controllable valve. A carburetor coupled to the engine air intake is set for an air/fuel ratio leaner than desired. The purge rate is regulated by electronically adjusting the valve in response to an EGO sensor. By regulating the purge flow rate, allegedly, the desired air/fuel ratio is achieved.
U.S. Pat. No. 4,677,956 issued to Hamburg discloses a fuel injected engine coupled to a fuel vapor recovery system. The fuel injector is regulated in response to an EGO sensor to achieve the desired air/fuel ratio.
The inventor herein has recognized a problem with fuel injected engines coupled to fuel vapor recovery systems wherein the air/fuel ratio is regulated in response to an EGO sensor. The problem is that when inducting evaporative fuel vapors at low engine loads, the fuel charge desired from the fuel injectors to achieve a desired air/fuel ratio may be below the linear range of the fuel injectors. That is, the amount of fuel required from the fuel injectors while purging fuel vapors at low engine loads may be so small that it is below the linear range of conventional fuel injectors. This situation is more likely to occur in multiport fuel injected engines (one fuel injector coupled to each combustion chamber rather than a single fuel injector coupled to the engine intake) wherein the pulse width of each multiport injector is considerably less than that required by a single main injector. Since, under the operating conditions described above, the relationship between fuel delivered and pulse width is nonlinear, accurate fuel control and accordingly accurate air/fuel ratio control is not obtainable.
The approaches described above, apparently, did not have to consider this problem since those approaches generally do not purge fuel vapors when the inducted airflow is below a threshold. Stated another way, prior approaches have only purged fuel vapors when the mass airflow was above a threshold to minimize the effect of purged fuel vapors upon the air/fuel ratio. The inventors herein have recognized that it is desirable to purge fuel vapors as frequently as possible including the purge of vapors during idle. Further, future government regulations may further limit the atmospheric discharge of fuel vapors thereby requiring the purge of vapors during idle and low engine loads. The prior approaches, however, will not achieve a desired air/fuel ratio when purging at idle or low engine loads.